The detection of a ground fault at an early point of time is an important measure in connection with such a multifurnace installation with several induction furnaces in order to improve the operational security of such a multifurnace installation. For this, ground-fault detecting means are used. With these means a measurement is carried out according to which a resistance value is ascertained by means of an own current source. An analogous output signal of the apparatus can be visualized by means of a pointer instrument and a programmable threshold can generate an alarm message.
Furthermore, such ground faults can be detected through a ground-leak sensor by means of a ground-leak sensor. Here, a passive measurement is carried out according to which the current flowing to ground is measured through a current transformer. An analogous output signal of the corresponding apparatus can be visualized by means of a pointer instrument and an adjustable threshold can generate an alarm message.
In the above-described apparatus of the prior art a ground-fault sensor associated with each induction furnace and connected to the electrical supply line to the induction-furnace coil as well as a ground-leak sensor connected to the at least two inverters are used. The furnace-related ground-fault sensors have to be switched off in such an apparatus as soon as at least one furnace is operated since they influence toward each other so that a satisfying release is not possible. Then, the ground-leak sensor takes over the function of detecting a ground fault. A furnace isolated from the power supply is further monitored by the ground-fault sensor since the same is not switched off with a separated furnace. If special requirements for instance a too wet crucible) exist the ground-fault sensor has to be deactivated with a key switch in order to be able to operate the corresponding furnace.
The provided ground-leak sensor enables a quick reaction on defects caused by electrical sparkovers arcing). Here, the leakage current is measured by means of a current detector wherein the detector lets pass high frequency current that is typically associated with arcing caused by insulation breakdown in the induction furnace refractory or the insulation that surrounds the coil while rejecting lower frequency voltage components. If the arcing current exceeds a preset threshold a trip signal is generated to shut down the furnace and an alarm is provided to signal the ground leakage fault to the furnace operator. Preferably, the furnace is switched off. However, one came to know that the ground-leak sensor does not operate in a satisfying manner in each case with small furnace voltages.